Methods known in the prior art for rear-side contacting wafer-based silicon solar cells are based on lithography, masking, and “lift-off” methods but are not suitable for industrial implementation for large-area application to thin-film solar cells because of the complexity, the handling outlay, and the low achievable machining speeds.
By contrast, the use of lasers for producing contact systems is suitable for producing large-area thin-film solar cells. Various process steps such as removing material in the form of points or lines, material alterations or laser doping by firing contacts are known in the prior art.
For example, in U.S. Pat. No. 5,538,564 A, p and n contacts having a high aspect ratio are produced in the active layer of a solar cell by doping using a pulsed laser in a gas atmosphere containing the dopant. This laser may be selected from the group of excimer, dye or YAG lasers.
A further important step for producing a rear-side contacting system for thin-film solar cells involves contact structures produced using the laser, including in particular holes in an insulation layer made in a rear side, which are required for connecting metal contacts and active solar cell layers. DE 690 12 517 T2 discloses a method for forming through-holes in a polyimide substrate, in which method holes are produced in a polyimide substrate in an oxygen-containing atmosphere using a defocused carbon dioxide laser of a defined energy density, and polyimide residues are removed by subsequent chemical etching. This publication makes reference to other known methods of laser drilling. Thus, in the case of the excimer laser, photodecomposition using electromagnetic energy in the UV range of the spectrum is used so as to break the chemical bonds in the substrate and decompose the polyimide. Using an argon ion laser, electromagnetic energy of this type is introduced into the polyimide substrate, and damages but does not decompose the film. This is followed by an etching step using plasma, so as to remove the polyimide damaged by the radiation. However, this etching step produces holes having different diameters or deviant hole shapes, and this is undesirable. If the holes are “drilled” using a carbon dioxide laser, this results in considerable amounts of residues in the form of ablated particles, which would have to be removed again by way of an etching step.
In the method disclosed in DE 100 05 330 A1 for producing high-resolution transparent and conductive structures, laser radiation acts on selected regions of a non-transparent, non-conductive layer, converting the material in the irradiated regions into transparent, conductive material. Once the conversion into a transparent material is complete, the laser radiation can also act on deeper layer regions. After the action of the laser radiation, there are transparent, conductive regions and non-transparent, insulating regions, which have different etching properties, in other words are affected to different extents, resulting in structuring.
In the method disclosed in DE 10 2007 051 725 A1 for contacting solar cells, a masking layer is opened to form narrow structures using laser radiation. If the substrate surface is damaged in this process, the recombination of generated charge carriers is increased there. To remove damage of this type, the surface is subjected to wet-chemical or plasma etching.
DE 199 15 666 A1 discloses a method for selectively contacting solar cells, in particular for contacting the emitter layer and/or base layer of the solar cell. In this method, a laser beam is directed onto an array of optical micro-lenses, the focal points of which are positioned in the region of a dielectric layer that covers the surface of the solar cell to be electrically contacted. At the focal points, material is removed as a result of the exposure until the surface to be electrically contacted has been uncovered there. Subsequently, the uncovered surface is metallized through the dielectric layer.
In DE 10 2009 057 881 A1, too, structures are produced using laser radiation. Here, an absorber layer that absorbs laser radiation is applied to a layer transparent to laser radiation. Local regions in the absorber layer are removed using laser radiation; subsequently, the now uncovered regions of the transparent layer are removed by means of an etching step.
WO 03/019674 A1 discloses directly producing contact holes through a layer or a plurality of layers as far as into the emitter layer or absorber layer using laser drilling, on the entire surface of a solar cell module, in connection with a series connection of the solar cells in a module. In this case, one layer is an insulating layer formed from two sub-layers, one sub-layer in turn being a synthetic resin layer.
WO 2010/012259 A2 discloses producing a heterocontact silicon thin-film solar cell in which the contacts for the emitter and the absorber reach different depths and extend in holes having lateral insulation, and the contacts may consist of TCO layers.
A method for producing a rear-side contact for polycrystalline thin-film solar cells was disclosed by M. Green et al. in Solar Energy 77, 2004, pp. 857-863. On an insulating synthetic resin layer, in a two-step process, an etching solution is applied in succession by inkjet printing to the locations at which the contact holes are to be produced as far as the absorber layer or as far as the emitter layer by etching. U.S. Pat. No. 7,585,781 B2 explains this method in detail; in particular, the insulation of the uncovered edges of the emitter layer by a “reflow” process of the material of the insulation layer is disclosed. The invention starts from this prior art. For completeness, reference should be made to the WO application having publication number WO 2005/024927 A1, on which the US patent is based and in which an organic insulation layer is also removed, inter alia, by laser ablation, before holes are etched as far as into the silicon absorber layer. However, the document also discloses the use of photographic techniques in which no laser is used.